1. Field of the Invention
This invention relates to an alarm system for detecting a sound and raising an alarm.
2. Description of the Related Art
Various alarm systems have been developed in order to prevent the burglary of automobiles and the invasion of an intruders. The detection means for detecting an alarm condition used in said alarm systems include ultrasonic waves, laser beams, heat sensors and the use of sound. According to the detection method using sound, when operations such as knocking, boring, breaking, rupturing, etc., windows, doors, walls, etc., occur, a sound having specific frequency components inherent to the materials is generated in accordance with the conditions of the material. Accordingly, it is possible to detect whether or not an alarm state occurs, by detecting and analyzing the resulting sound. For instance, an alarm system that stores, in advance, a glass smashing sound of a window pane of an automobile or a room and raises an alarm when the said sound is actually generated is known. This glass smashing alarm system has a construction wherein a microphone disposed internally picks up the frequency (e.g. 4.7 KHz) inherent to glass smashing, and said act is judged as occurring when this frequency is detected. This system is used as a burglar-proof system of automobiles, for example.
The glass smashing alarm system described above inadvertently operates, however, when a different sound having the same frequency components as that of glass smashing occurs. It also operates erroneously when vibration having a frequency close to that of the glass smashing sound is directly transmitted to an acoustic-electric convertor of the microphone. Such erroneous operations are likely to occur particularly when the microphone is disposed inside an instrument panel of the automobile.
The present invention is aimed at reducing the possibility of occurrence of an erroneous alarm operation by considering, in particular, the generation of an artificial glass smashing sound with the passage of time.